Isotopic mass and lattice constant: X-ray standing wave measurements

Isotopic effect in extended x-ray-absorption fine structure of germanium

Extended x-ray absorption fine structure has been measured on two powdered samples of (70)Ge and (76)Ge as a function of temperature from 20 to 300 K. The effect of isotopic mass difference on the amplitude of relative atomic vibrations is neatly evidenced by the temperature dependence of the difference of Debye-Waller factors. The isotopic effect is also detected on the difference of nearest-neighbor average ineratomic distances, thanks to a resolution better than 10 fm.

Anomalous isotopic effect on the lattice parameter of silicon

The difference Delta(a)=a(30)-a(28) of the lattice parameter of 30Si and 28Si crystals is measured over a temperature range from 4.7 to 700 K. In disagreement with existing knowledge, the strongest isotopic effect is not detected at the lowest achieved temperature T=4.7 K. An anomalous behavior is observed: The relative difference |Delta(a)/a| attains its maximum value of 56.8(5) ppm at T=75(10) K. The anomalous behavior is attributed to the influence of phonon modes with negative Grüneisen parameters. At T=700 K the effect still amounts to 30% of the maximal value. The experimental data are consistent with an approach based on the density-functional perturbation theory.

X-ray standing wave analysis of the effect of isotopic composition on the lattice constants of Si and Ge

The x-ray standing wave (XSW) technique is used to measure the isotopic mass dependence of the lattice constants of Si and Ge. Backreflection allows substrates of moderate crystallinity to be used while high order reflection yields high accuracy. The XSW, generated by the substrate, serves as a reference for the lattice planes of an epilayer of different isotopic composition. Employing XSW and photoemission, the position of the surface planes is determined from which the lattice constant difference Deltaa is calculated. Scaled to DeltaM = 1 amu we find (Deltaa/a) of -0.36x10(-5) and -0.88x10(-5) for Ge and -1.8x10(-5) and -3.0x10(-5) for Si at 300 and 30 K, respectively.

Modified magnetism at a buried Co/Pd interface resolved with X-ray standing waves

Soft x-ray standing waves produced by a multilayer interference substrate add depth sensitivity to magnetic circular dichroism to resolve changes in Co magnetism across a 1 nm distance from the Co center to the Co-on-Pd interface of a Pd/Co/Pd trilayer with an in-plane magnetization. Large enhancements of the number of Co d holes, and of in-plane orbital and spin magnetic moments, are strongly localized at a thin, chemically modified interface layer. These results provide new insight into magnetic anisotropy at interfaces, and suggest a broad applicability of such standing wave measurements to interface magnetism studies.